benignuman –
It is possible that someone might disagree with the Rambam as to the status of this din, and rather holds it was derived in an honest attempt to derive the meaning of the Torah. I don’t think that would be apikorsus. But saying that the Torah was meant literally and the Sages changed the law because they didn’t like it and then lied and said it was what the Torah really meant, is apikorsus.
But would you say it is apikorsus in the sense that every mevazeh talmidei chachamim is an apikores (if one says this derisively) or because there is something here which is a fundamental denial of Torah sheba’al peh? If the latter, how do you support this view, considering Sumchos held that tzroros pays nezek shalem? To put it a little differently, if someone holds this view and frames it a in a non-condescending but respectful way, and not in all cases, is he an apikores?